The present invention relates to a device for heating an aerofoil. The aerofoils in question are generally those the aerodynamic shape of which has not to be disturbed by the formation of ice, especially helicopter blades (main rotor or counter-torque rotor) or alternatively aeroplane wings, etc.
The problem of the icing-up of the aerofoils is well known in the aeronautical industry. The shape of the aerofoils may be altered owing to the formation of ice resulting in the fact that during flight the aerofoil encounters droplets of super cooled water contained in the atmosphere.
This problem is often dealt with by equipping the aerofoil with a Joule-effect heating structure. A distinction is made between "de-icers", having heat-dissipating resistive elements powered intermittently in order to eliminate the ice which regularly forms, and "anti-icing devices", whose resistive elements are powered continuously in order to prevent ice from forming. The present invention may be applied to de-icers and to anti-icing devices.
The heating structure usually consists of metallic resistors. These metallic resistors pose problems of mechanical strength, particularly in the case of an aerofoil made of composite material, of resistance to defects (multiple redundancy is needed in order to guard against breakage of a metallic filament preventing the entire device from operating), of nonuniformity of the surface density of the heating, and of corrosion.
In order to limit the impact of these problems, it has been proposed to use a composite de-icer, whose resistive elements are composed of carbon fibers (see U.S. Pat. No. 4,737,618). The composite structure of the de-icer is constructed laid out flat: plies of uni-directional carbon fibers are dispensed and interposed between insulating and support fabrics made of fiberglass. The carbon plies and the glass fabrics are impregnated with a thermosetting resin. The de-icer made laid out flat is then shaped in an appropriate mold where a heating cycle is applied in order to set the resin. This method is suitable for the production of a de-icer the definitive shape of which is not too complicated, for example a de-icer intended to equip the main blade section of a wing or blade whose leading edge is straight or approximately straight, the uni-directional carbon fibers being arranged parallel to the leading edge. However, for an aerofoil of complicated shape, it is difficult to exercise control over the spatial distribution of the uni-directional carbon fibers, which arises when the de-icer is shaped. There is the risk, on the one hand, of obtaining build-up of conducting fibers over some portions of the chord, which results in a problem of local overheating, and on the other hand of obtaining other portions which are lacking in carbon fibers, which results in a problem of local icing-up. In particular, if the region near the leading edge is convexed in two directions at some points, then there is a risk of having gaps in the carbon fibers in the immediate vicinity of the leading edge, whereas this is where the need for heating is the greatest. This problem is encountered especially at the end of the main blade section of a blade or of a wing.
For such aerofoils of complicated shape, one solution might be to provide several layers of uni-directional fibers not parallel to the leading edge but crossed with each other. However, the drawback of such a solution is that it would produce non-uniform heating. In particular, this solution would lead to conducting elements of different apparent lengths being arranged along the surface of the aerofoil, which would cause variations in resistance from one element to another.
One object of the present invention is to propose a heating device based on carbon fibers (or on other conducting fibers that can be integrated into a composite structure) which is easy to handle and which can, especially, be used on aerofoils of complicated shape.